Many forms of thickness measuring gauges are available with localized measurement devices, such as a pair of calipers, being a typical example. Sometimes it is important to determine the overall flatness of a planar sheet, and in those cases use of a localized measuring device such as a set of calipers is not entirely satisfactory because the localized measurements do not give an indication of the gross variations from planarity which are characteristic of a sheet of uniform thickness but nonuniform flatness.
Flatness testing apparatus of various configurations is also available. Such devices can rely on numerous technologies; for example, flatness testers have been configured utilizing optical principles, pneumatic principles, principles of mechanical contact, proximity and the like. While such devices are typically suited to their particular needs, one application where a suitable device is not currently known to be available is in the steel producing industry where steel sheets or stampings are to be produced which not only have a thickness tolerance but also an allowable flatness tolerance. While the steel production environment is not known for its cleanliness or quiet, it is often desirable to run the necessary quality control tests on the produced sheets right in the production environment, and by a production worker.
Flatness tests have been conducted in that environment by mechanical means involving the use of a surface plate and feeler gauges The surface plate is a plate presenting a flat surface on which the sheet to be measured is placed, and an operator manually attempts to insert feeler gauges between the sheet and the surface plate. If it is possible to insert a feeler gauge of a predetermined thickness between those elements at their point of widest separation, it is determined that the sheet is of unacceptable flatness and the sheet is rejected. However, it can well be appreciated that such a procedure is subject to substantial error and variability since it is up to the operator to determine the point at which the feeler gauge is to be inserted, the operator has the opportunity to vary the force at which he attempts to insert the feeler gauge, the sheets are typically of relatively thin flexible material and mechanical deformation by the feeler gauge or otherwise could affect the result, etc. Thus, the mechanical test is unacceptable in terms of difficulty and time consumption, and is also operator sensitive and subject to error and unacceptable reliability.